The invention relates to methods of transmitting encoded images in inactive video periods of an analog (i.e. PAL, NTSC, etc.) television signal and reproducing said images in synchronism with given frames of the television signal.
A known method of transmitting encoded images in inactive video periods of an analog television signal for reproduction in synchronism with given frames of the television signal is provided by World System Teletext. Teletext, standardized in the early 70s, provides the facility to transmit subtitle pages along with other teletext pages in the vertical blanking interval of the television signal. The subtitles are transmitted in synchronism with the video scene. That is, each subtitle page is inserted in the teletext stream precisely at the time it is to be reproduced. Since teletext information is decoded in real time, the subtitle page is displayed without any delay.
More complex data service systems such as TeleWeb are currently being developed. TeleWeb images are images encoded in accordance with the HTML standard and transmitted in the vertical blanking interval of a television signal. The delay between transmission and display of a TeleWeb image is not known in advance and may vary from decoder to decoder. The unknown delay of a TeleWeb subtitle image poses a problem in that synchronization between a television program and its subtitling is difficult to achieve. This is especially the case when television program is being subtitled in several languages. The bandwidth of the system (one or few lines of the vertical blanking interval) is not sufficient to transmit all subtitles in real time.
It is an object of the invention to overcome the above-mentioned problems.
To this end, the invention provides a method of transmitting an encoded image as defined in claim 1, a method of reproducing an image as defined in claim 4, a television receiver as defined in claim 7, and a television signal as defined in claim 8.
The invention provides advance transmission of encoded images, with a time stamp specifying the desired time of display being accommodated in the image data. The time stamp is related to a reference clock, samples of which are also transmitted in inactive video portions of the television signal. Preferably, the reference clock is the clock time which is transmitted in the page headers of a teletext service or in teletext packets 8/30. It is thereby achieved that no additional transmission capacity is required for transmission of the reference clock.
It should be noted that advance transmission of subtitles with time stamps is known per se from International Patent Application WO 96/19077. In this prior-art system, the subtitles are transmitted along with a compressed digital video signal. Digital video signals already include a reference clock. Digital television receivers comprise an input buffer in which the compressed video signal is buffered in order to compensate for the varying amounts of frame data. The encoded video frames include time stamps representing the time at which they have to be removed from the buffer and/or displayed, so that the buffer does not overflow or underflow. The transmitted reference clock thus controls the video display timing. As the time stamps in subtitle images relate to the same reference clock, the subtitles can exactly be synchronized with given frames of the video signal.
The teaching of WO 96/19077 cannot be applied to conventional analog television systems because the frames of an analog video signal are not identified by time stamps. Exact synchronization of a subtitle and a given frame as offered by digital systems can therefore not be achieved. Instead, the invention provides an estimation of the time at which the given frame will be displayed in terms of an external reference clock. The reference clock is transmitted along with the television signal but has no relation with its video contents. It has been found that the method provides sufficient accuracy of synchronization.